Solid state imaging devices, including charge coupled devices (CCD), CMOS imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit includes a focal plane array of pixel cells or pixels, each one including a photoconversion device, which may be a photogate, photoconductor, or a photodiode having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device such as a transistor for transferring charge from the photoconversion device to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
FIG. 1 is an electrical diagram of a conventional pixel cell 10. As illustrated in FIG. 1, the pixel 10 includes a photoconversion device 50. The photoconversion device 50 is illustratively a photodiode and may be a p-n junction photodiode, a Schottky photodiode, or any other suitable photoconversion device. There is an electrical connection between photodiode 50 and a supply voltage (VAA-PIX) through transistor 25. Transistor 25 is depicted as an anti-blooming transistor, which can be operated to allow excess collected charge to drain to VAA-PIX.
The pixel 10 also includes a transfer transistor 26 for transferring charge from the photoconversion device 50 to the floating diffusion region 16. Floating diffusion region 16 is coupled to a source/drain region of reset transistor 18, which operates to reset the floating diffusion region to a predetermined voltage, e.g., VAA-PIX. The floating diffusion region 16 is also coupled to the gate of a source follower transistor 40, which receives the charge from the floating diffusion region 16 and provides an output signal based on the stored charge to a source/drain terminal of a row select transistor 42. The row select transistor 42 operates to couple the signal produced by source follower transistor 40 to the column line 31 where it is read out.
Pixel 10 can be included in an array of pixels within an imaging device (not shown). Typically, pixels 10 in imaging devices are tested by the manufacturer and circuitry adjustments are performed if necessary. Conventional testing is time consuming and requires analysis of the performance of the imaging device when exposed to a light source. In addition to the time needed for optical testing, significant time is needed to set up conventional testing equipment. Further, with conventional testing techniques, the pixels 10 of the imaging device can not readily be tested by end users of the device to make adjustments over the life of the imaging device. Accordingly, there is a need for a testing methods and apparatuses for an imaging device that may be performed without a light source and by end users.